1. Field of the Invention
The present invention relates to a switching control circuit integrated into an IC (integrated circuit), and also relates to a switching power-supply apparatus including the switching control circuit.
2. Description of the Related Art
Typically, a switching control IC in a switching power-supply apparatus includes a circuit arranged to provide various functions for, for example, output control, startup operation, overcurrent protection operation, overvoltage protection operation, standby operation, and power-factor correction operation. The number of functions is increased to improve the functions of the switching control ICs.
In order to set the functions of a switching control IC so that they correspond to operation specifications of an application, multiple terminals arranged to interface with external circuits are required for the respective functions. Thus, when the number of functions increases, the number of terminals increases accordingly. An increase in the number of terminals results in an increased package size of the switching control IC, which leads to an increase in the unit price of the IC.
On the other hand, a switching control IC having a reduced number of terminals is required in order to meet demands for miniaturization of a switching control IC and reduction of the mounting dimension thereof. Sixteen pins are often provided as the terminals for typical switching control ICs and eight pins are often provided for small-sized switching control ICs. Four pins are also provided for further miniaturized switching control ICs.
When the number of terminals for a switching control IC is limited, the number of functions that can be provided are also limited. Thus, it is necessary to produce a wide range of variants of ICs in accordance with the functions and to selectively use the ICs depending on the specifications and the applications. In this case, there are problems in that the number of variants of the ICs increases, not only the manufacturing process but also the IC management becomes complicated, and consequently the unit price of the ICs increases.
It is inevitable that the size of the IC will increase as the required functionality increases. However, recently, even for small-sized switching control circuits, the number of required functions has been increasing. Under the current situation, mass production of a small number of variants is effective in order to reduce the unit price of semiconductors, such as ICs. In addition, a small-sized IC package that includes a smaller number of terminals can also reduce the unit price of the IC.
Japanese Unexamined Patent Application Publication No. 2007-73954 discloses a technology for a switching control IC directed at reducing the number of terminals.
FIG. 1 is a circuit diagram of a switching power-supply apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2007-73954. In FIG. 1, a switching power-supply apparatus 101 includes a transformer 105 including a primary winding 127 and a secondary winding 129, a rectifying/smoothing circuit including a diode 117 and a capacitor 119, a feedback circuit including a Zener diode 121, a photocoupler 113, and a resistor 123, and an integrated circuit 103.
The integrated circuit 103 is connected to the primary winding 127. The integrated circuit 103 is a switching regulator including an internal switch coupled between a drain (D) terminal and a source (S) terminal of the integrated circuit 103.
During operation, the internal switch in the integrated circuit 103 adjusts energy transmission from an input 107 to an output 109 through the transformer 105. A feedback signal is input from the feedback circuit to the integrated circuit 103.
A multifunction capacitor 111 is connected to a bypass terminal BP of the integrated circuit 103. The multifunction capacitor 111 is arranged to provide a power-supply decoupling function for the integrated circuit 103 during normal operation. The internal circuit of the integrated circuit 103 receives power or bias current from the multifunction capacitor 111 to operate during normal operation while adjusting the output 109.
The multifunction capacitor 111 is arranged to select a parameter/mode of the integrated circuit 103 during an initialization period of the integrated circuit 103. During the initialization period, the parameter/mode of the integrated circuit 103 is selected.
In the switching power-supply apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2007-73954, a terminal arranged to provide a necessary function is used to determine the setting of another function during an initialization period. Thus, the capacitance of the multifunctional capacitor is set so that the parameter/mode of the integrated circuit 103 is selected in accordance with a difference in the rate of an increase in the charging voltage during the initialization period.
With such a configuration in which the setting of the parameter/mode of the switching control IC is determined during the initialization period, the number selectable functions is relatively small. In addition, since only the capacitor is used to set the parameter/mode, the selection function is severely restricted.